[Frontend] early return chat format resolution when specified

[xzbdmw]

The auto-detection of content format introduced in #9919 involves Jinja parsing, and it is time-consuming for small models and short max_tokens, such as Qwen3-0.6B with max_tokens=1. Skipping this auto-detection can provide a 3X RPS boost when using the --template-content-format string flag in L20.

[github-actions]

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[gemini-code-assist]

Summary of Changes

Hello @xzbdmw, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a performance bottleneck identified in the chat template content format auto-detection logic. By allowing users to explicitly specify the content format, the system can avoid unnecessary and computationally intensive Jinja parsing, leading to improved throughput and efficiency, particularly in scenarios where auto-detection overhead is significant.

Highlights

• Performance Optimization: Introduced a mechanism to explicitly skip the time-consuming auto-detection of chat template content format, which involves Jinja parsing. This can significantly boost Requests Per Second (RPS) for specific use cases, such as small models with short max_tokens.
• API Usage Enhancement: Users can now leverage the --template-content-format string flag to bypass the automatic detection, gaining direct control over the content format resolution process.

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[gemini-code-assist]

Code Review

This pull request fixes an issue where the auto-detection of chat template content format was not being skipped when an explicit format was provided. The changes are well-targeted and logically sound, leading to the described performance improvements.

[aarnphm]

Thanks.